Vacuum seal for a thin beryllium x-ray window



Dec. 31, 1968 3,419,741

VACUUM SEAL. Fon A THIN BERYLLIUM x-RAY wINnow V. J. LEGENDRE FiledApril 19.4 1966 FIG. 2

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INVENTOR VICTOR J LEGENDRE United States Patent O 3,419,741 VACUUM SEALFOR A THIN BERYLLIUM X-RAY WINDOW Victor J. Legendre, Mountainside, NJ.,assignor to Thomas Electronics, Inc., Passaic, NJ., a corporation of NewJersey Filed Apr. 19, 1966, Ser. No. 543,712 7 IClaims. (Cl. 313-59)ABSTRACT oF THE DISCLOSURE The invention pertains to a permanentvacuum-tight seal for beryllium windows used in X-ray apparatus. Thesealing members allow for unequal expansion of parts without unduestress.

This invention relates to a support which has the characteristic ofcompensating for differentials in thermal expansion and moreparticularly to an improved support for permanently sealing a window inan X-ray device.

While thereare undoubtedly many areas in which a support possessing thecharacteristic of compensating for differential thermal expansion can beutilized, I have found the application of the support to a window in anX-ray device to be of particular significance.

Since the discovery of X-rays, one of the major problems in thedevelopment of the X-ray tube art has been the transmission of X-raysthrough the tube wall. X-ray tube housings are constructed so as tocontain all of the X-radiation with the exception of the particular coreof radiation permitted to escape from the housing through a low X-rayabsorption window.

The search for an efficient low X-ray absorption window began when A. F.and F. A. Lindemann devised the so-called Lindemann glass. Since thenvarious other materials have been used for X-ray windows, such ascellophane, aluminum foil, and beryllium. The advantage of usingberyllium possessing a low specific density and one of the lowest massabsorption coefficients per unit thickness is well known. Beryllium,however, also possesses a much higher coefficient of linear thermalexpansion than glass. This variance in linear thermal expansion presentsa majorobstacle to the making of vacuum-tight seals of thin beryllium toglass envelopes, due to the likelihood of resulting breakage of theglass and seal. Consequently, permanent beryllium to glass seals capableof withstanding the large temperature Variations of tube processing havebeen difficult to obtain. The severity of the problem is even greater asthe diameter of the window increases.

While demountable mechanical seals for beryllium windows, utilizing softmetal gaskets to provide vacuum tightness, have been used in the pastwith some degree of success, they possess the disadvantages of highcost, consumption of space and lack of reliability.

In accordance with the present invention, these and other disadvantagesof the prior art are overcome by the provision of a support providing apermanent vacuumtight seal in which differences in thermal expansionbetween the materials sealed are attenuated.

Accordingly, it is a principal object of the present invention toprovide a permanent seal which compensates for differentials in thermalexpansion.

It is an object of the present invention to provide a support for awindow in a housing which compensates for the difference in thermalexpansion between the window and the housing.

It is an object of the present invention to provide a seal between awindow having a high coeiiicient of ex- 3,419,741 Patented Dec. 31, 1968ICC pansion and a body having a comparatively low coefficient ofexpansion.

It is an object of the present invention to permanently seal a largethin beryllium disc in a soft glass envelope.

It is an object of the present invention to provide a permanentvacuum-tight seal for a beryllium window in a soft glass envelopewithout the use of nuts and bolts and without the use of secondary sealsor without the necessity of placing the beryllium behind another glassor mica panel.

It is an object of the present invention to provide a permanent seal fora beryllium window in a glass envelope which will withstand heat cyclingfrom 0 to 425 C.

It is an object of the present invention to provide a permanentvacuum-tight seal for a beryllium window in a glass envelope wherein thesealing alloys and the geometrical design of the mating parts absorb thethermal expansion from beryllium to soft glass.

It is another object of the present invention to provide a permanentvacuum-tight seal capable of attenuating a coeicent of thermal expansionfrom 150 X106 to 89 10TH.

It is another object of the present invention to provide a permanentvacuum-tight seal for a beryllium window in a glass envelope wherein thematerial comprising the portion of the support providing the seal indirect contact with the beryllium has a coefficient of expansionapproximately equal to the coefcient of expansion of the beryllium andthe material comprising the portion of the support providing the seal incontact with the glass envelope has a coefficient of expansionapproximately equal to the coeicient of expansion of the glass envelope.

It is another object of the present invention to provide a permanentvacuum-tight seal for a beryllium window in a glass envelope asdescribed in the preceding paragraph wherein the expansion is attenuatedby converting the linear expansion from the major axis of the berylliumwindow to the minor axis of the glass envelope through one or more 90bends and long lines.

Other detailed objects and advantages of the present invention willbecome apparent from the following description of the present embodimentthereof, taken in conjunction with the drawings which accompany and formpart of the specification.

In the drawings:

FIGURE 1 shows an embodiment of the present invention in a colortelevision tube.

FIGURE 2 is a fragmentary sectional view of a structure made inaccordance with the invention; and

FIGURE 3 is a plan view of the structure shown in FIGURE 2.

As shown in the drawings, a seal is provided for a window in a body,wherein the body has a low coefficient of expansion as compared to thewindow.

Referring to the drawings, numeral 10 denotes a beryllium window sealedin a glass envelope 12, such as a cathode ray tube used in colortelevision. Although the glass used for TB tube envelopes providessatisfactory shielding from X-rays for operators and spectators, thethermal expansion of the glass is no match for a beryllium window. Thecoefficient of thermal expansion of glass is 89x10-6 las compared to150)(106 for beryllium. This is a 60% mismatch. Obviously, in order topermanently seal the beryllium window in the glass envelope it isnecessary to provide some intermediate means for absorbing this highdifferential and thus avoid excessive strain and glass breakage duringprocessing heat cycles of 30 to 400 C. I have found that breakage of theglass and the seal can be prevented by a seal having a geometricaldesign capable of absorbing the thermal expansion for beryllium to softglass.

In my improved structure I have provided a frame 14 for mounting theberyllium window in the glass envelope 12. The frame 14 is of such aconfiguration as to attenuate the difference in thermal expansionbetween the envelope and the window by converting the linear expansionfrom the major axis of the window to the minor axis of the envelopethrough one or more 90 bends and long lines.

The envelope 12 has an opening 16 therein. An annular recess 18 isprovided about the opening 16 on the outer side of the envelope in whichrecess the frame 14 is mounted supporting the window across the opening.

The frame 14 has a circular fiange 20 with a central opening therein. Athin sheet beryllium member 10 is mounted across the opening and has itsedges overlapping and sealed to the circular fiange 20.

The frame 14 is composed of a member 22 having a coefficient ofexpansion substantially the same or at least -compatible to that of thewindow 10 and a member 24 having a coefficient of expansionsubstantially the same or at least compatible to that of the envelope12.

In the present embodiment of the invention employing a beryllium window10, the member 22 is made of a cupro-nickel alloy of the Monel typewhich has a thermal coefficient of expansion approaching that ofberyllium. The beryllium window is sealed to the circular flange 20 ofmember 22 with a vacuum-tight seal by brazing.

The member 24 on the other hand is made of a chromenickel-iron alloysuch as Sylvania #4 which has a ther mal coefficient of expansioncompatible with TV bulb glass. Member 24 is sealed in the annular recess18 of the glass envelope with a vacuum-tight seal by a glass frit seal28.

One of the essential features of the present invention is theconfiguration of frame members 22 and 24. Frame member 22 extendsoutward from the circular flange 20 and the surface of the envelope on aline perpendicular to the major axis of the window and the envelopesurface, a distance at least equal to the radius of the beryllium seal.Member 22 is then provided with a right angle extending outward from itsown central axis on a line parallel to the major `axis of the window andthe envelope surface until it comes in contact with member 24 extendingoutward from the recess of the envelope 18 and the frit seal 28 alongthe minor axis of the envelope surface at which point member 24 takes aright angle in order to conform with the surface of member 22, bothmembers then join in a right angle extending outward from their owncentral axis and then outward perpendicular to the major axis of theenvelope surface. Members 22 and 24 are heliarc welded together alongtheir juncture 30 forming a vacuum-tight seal. It is noted that thejuncture of members 22 and 24 is out of contact with the shoulder of thefrit seal 28 allowing maximum flexability of the frame.

The extension of the window frame along the minor axis of the surface ofthe beryllium window and the surface of the envelope, or in other wordsin a direction perpenidcular to the major axis of the beryllium windowand envelope surface, in combination with the matching of the thermalcoefiicient of expansion of frame members to the window and envelope aswell as perfect annealing, provides for the absorption of the differencein thermal expansion between the glass envelope and the berylliumwindow.

Another factor that contributes to the success of my invention is thefact that tubes in which the structure is employed are normally undervaccum. Since the longest portion of the attenuation path is along theportions of the frame member extending outward perpendicular from theenvelope surface, the vacuum tends to further attenuate the thermalexpansion along this path.

While the present embodiment of the invention illustrated relatesparticularly to the sealing of a window having a high thermalcoefficient of expansion in an envelope having a low thermal coeicientof expansion, my invention can be easily adapted to a window having alow thermal coefficient of expansion in an envelope having a highthermal coefficient `by providing a frame having members withinterchanged characteristics so that the thermal expansion of the majoraxis of the envelope will be attenuated and applied to the minor axis ofthe Window. I am aware that many other modifications thereof may be madewithout departing from the spirit of the invention. It is thereforeappropriate that the appended claims be accorded that broadinterpretation as is consistent with the spirit and scope of theinvention.

What is claimed is:

1. A support providing a permanent vacuum-tight seal for a window havinga substantially different thermal coefficient of expansion than theenvelope in whose aperture it is mounted, comprising va frame meanshaving a first `and second member, the first of said members having anopening therethrough, said window extending across said opening with theperipheral edges thereof conforming with and sealed to the surface ofsaid mem- -ber forming said opening, said first member havingsubstantially the same coefficient of expansion as said window, aperipheral edge of said second member conforming with and sealed to thesurface of said envelope aperture, said second member havingsubstantially the same coefficient of expansion as said envelope, saidfirst and second members being joined together in a vacuum-tight sealalong lan edge remote from said window and said opening, said framemeans being of such a geometrical configuration as to `attenuate thedifferences in thermal expansion between the window and the envelope.

2. A support providing a permanent vacuum-tight seal for a window havinga substantially higher coefficient of expansion than the envelope inwhose aperture it is mounted, -comprising a frame means having a firstand second member, the first of said members having a circular fiangewith a central opening therein, said window extending across saidopening with the peripheral edges thereof conforming with and sealed tothe surface of said flange, said first member having substantially thesame coefficient of expansion as said window, an annular recess aboutsaid aperture on the outer side of said envelope, a peripheral edge ofsaid second member conforming with and sealed to the surface of saidrecess, said second member having substantially the same c0- efiicientof expansion as said envelope, and said first and second members beingjoined together in a vacuum-tight seal along the edges remote from saidwindow and said envelope.

3. A support as defined in claim 2, wherein a portion of said firstmember extends outward from and perpendicular to the major axis of saidWindow and envelope surfaces a distance equal to at least half thedistance across the window.

4. A support as defined in claim 3, wherein at the end of the portion ofsaid first member extending outward from and perpendicular to the majoraxis of said window and envelope surfaces, said first member is providedwith a right angle extending outward from its own central axis on a lineparallel to the major axis of the window and the envelope surface untilit cornes in contact with said second member extending outward from saidrecess along the minor axis of the envelope surface, at which point bothmembers then join in a right angle extending outward from their owncentral axis and then outward perpendicular to the major axis of theenvelope surface.

5. An X-ray device including a sealed envelope, an aperture in saidenvelope, a window, a frame means having a first and second member, thefirst of said members having a circular ange with a central openingtherein, said window extending across said opening with the peripheraledges thereof conforming with and sealed to the surface of said flange,said first member having substantially the same coeficient of expansionas said Window, an annular recess about said aperture on the outer sideof said envelope, a peripheral edge of said second member conformingwith and sealed to the surface of said recess, said second member havingsubstantially the same coefficient of expansion as said envelope, saidrst and second members being joined together in a vacuum-tight sealalong their edges remote from said window and said envelope, and avacuum contained within said envelope cooperating with said frame memberso as to attenuate the differences in thermal expansion between thewindow and the envelope.

6. An X-ray device as defined in claim 5, wherein a portion of said rstmember extends outward from and perpendicular to the major axis of saidwindow and envelope surfaces a distance equal to at least half thedistance across the Window.

7. An X-ray device as defined in claim 6, wherein the peripheral edge ofsaid second member conforming with and sealed to the surface of saidrecess about said aperture on the outer side of the said envelope liesalong the minor axis of said window and envelope surfaces.

References Cited UNITED STATES PATENTS 2,665,391 1/1954 Bleeksma 313--593,115,957 12/1963 Heil. 3,134,903 5/1964 Fengler 313-59 X 3,243,0723/1966 Day.

JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

U.S. Cl. X.R.

